Immunosuppressive drugs are used in immunosuppressive therapy to inhibit or prevent activity of the immune system. Clinically they are used to prevent the rejection of transplanted organs and tissues (e.g. bone marrow, heart, kidney, liver) and in the treatment of autoimmune diseases or diseases that are most likely of autoimmune origin (e.g. rheumatoid arthritis, myasthenia gravis, systemic lupus erythematosus, Crohn's disease, and ulcerative colitis).
Single receptor assays for immunosuppressive drugs such as FK506 (tacrolimus) and rapamycin (sirolimus) have used immunophilins, e.g., FKBR's (FK binding proteins), but the FKBP's show significant cross-reactivity for more than one immunosuppressive drug or for inactive metabolites. Since immunosuppressive drugs are often given in combination with each other, specificity for the parent drug is critical, and the aforementioned assays are severely limited for use in routine therapeutic drug monitoring. This problem is overcome by the use of specific FKBP's for single receptor assays. Others have tried to solve this problem by developing single receptor assays using antibodies, i.e., immunoassays, which are specific for the immunosuppressive drugs of interest. Although the immunoassays show good selectivity for the parent drugs, they generally show significant cross-reactivity to one or more inactive metabolites. This cross-reactivity tends to give an undesirable positive bias in the immunoassays when compared to chromatographic reference assays. There remains a need for specific, sensitive, and stable assays for immunosuppressive drugs, and especially for such assays which can be applied to high throughput clinical analyzers.
FKBP25 is known to be rapamycin specific, but this property has not been previously exploited for a specific assay for rapamycin. FKBP's with high cross-reactivity to both rapamycin (sirolimus) and FK506 (tacrolimus) have been used in heterogeneous single receptor assays. Single receptor assays for immunosuppressive drugs in homogeneous or heterogeneous formats which utilize antibodies are known in the art.